1. Field of the Invention
The present invention generally relates to a voltage converter, in particular, to a buck-type voltage converter capable of generating an output signal having a steady ripple.
2. Description of Related Art
FIG. 1A shows a conventional voltage converter 100. Referring to FIG. 1A, the voltage converter 100 includes a driver 110, a logic operation circuit 120, a comparator CMP1, transistors M1 and M2, an inductor L1, a capacitor C1 and resistors R1 and R2. In the voltage converter 100, the comparator CMP1 compares a reference signal REF with a feedback signal VFB to generate a comparison result, which is processed by the logic operation circuit 120 and the driver 110 in sequence to generate driving signals DRV1 and DRV2. In addition, the transistor M1 receives a power supply voltage VIN.
In the voltage converter 100, when the comparator CMP1 determines through comparison that the feedback signal VFB is lower than the reference signal REF, the transistor M1 is turned on for a period of time according to the driving signal DRV1, and a buck output voltage VOUT is increased correspondingly. Afterward, the transistor M1 is turned off according to the driving signal DRV1 and the transistor M2 is turned on according to the driving signal DRV2, and when the buck output voltage VOUT drops until the feedback signal VFB is once again lower than the reference signal REF, the transistor M1 is turned on for a period of time again according to the driving signal DRV1.
When the equivalent series resistance (ESR) of the capacitor C1 is too low, refer to FIG. 1B, which is a waveform diagram illustrating operation of the voltage converter 100. The ripple component on the feedback signal VFB will become very unobvious, which incurs an unsteady ripple phenomenon on the buck output voltage VOUT, degrading the quality of the buck output voltage VOUT.